This converts the three direct buffer functions from JNI to internal VM
calls.  As a bonus, we grab PlatformAddress.osaddr directly instead of
retrieving it with PlatformAddress.toLong().

We're still calling through getEffectiveAddress(), which is where most
of the complexity lies.

Nudged a couple of comments.

We weren't always switching to "running" mode before playing with
objects.  Now checkObject() always changes modes, and things that called
checkObject while in "running" mode now do so after switching back to
"native" (which shouldn't matter now, but I don't want to require nested
switching to work if I don't have to).

Vaguely related to internal bug 1943379, since I think we want to do
some updates to CheckJNI there.

Also adds some "return" calls after abortMaybe(), since it won't actually
abort if "warnonly" is set.

Use memcmp instead of "==" so that, if the pointer is unaligned on a
platform where alignment matters, we fail with a nice warning instead
of a bus error.

Found while tracking down bug 1823434.